Injection nozzles have long been used to convert a continuous flow of a liquid under pressure into a continuous flow of solid particles. Frequently, carbon dioxide (CO.sub.2) is the medium which is transformed from liquid into solid particles through the use of such injection nozzles. This is particularly true when there is a need for a continuous flow of solid CO.sub.2 particles, such as is needed by particle blast apparatuses which utilize CO.sub.2.
Such apparatuses are well known in the art. U.S. Pat. No. 4,744,181 discloses an apparatus used for cryogenic cleaning in which a continuous flow of CO.sub.2 pellets are directed against an object to be cleaned. Such an apparatus requires a continuous supply of CO.sub.2 pellets in order to allow the operator to use the apparatus for continuous cleaning. While the CO.sub.2 particles may actually be crushed dry ice (solid CO.sub.2) created by the apparatus by crushing large blocks of dry ice, the cleaning effect of the apparatus is more efficient if a uniform supply of solid CO.sub.2 particles having a size and density within a controlled range is used. Crushing of dry ice results in unpredictable particle sizes and densities, thereby decreasing the efficiency of a cryogenic cleaning apparatus.
One method for supplying a flow of solid CO.sub.2 particles within the parameters necessary for the cryogenic cleaning apparatus is to convert liquid CO.sub.2 to the solid state and then to feed the particles directly into the apparatus. For maximum efficiency and economics, this requires that the actual phase change occur in close physical proximity to the cleaning apparatus. To accomplish this, a supply of pressurized liquid CO.sub.2 is caused to flow through an injection nozzle where it is converted to solid CO.sub.2 particles or CO.sub.2 snow. The solid CO.sub.2 is then forced through a die and formed into pellets by a breaker plate associated with the die. The pellets can then be transferred into the cryogenic particle blast cleaning apparatus in a continuous fashion.
Prior art injection nozzles have typically comprised a nozzle body having a single large orifice through which the entire flow of CO.sub.2 passes. The pressure of the CO.sub.2 drops as it flows through the orifice, causing the liquid CO.sub.2 to flash to the solid state. During this process, the temperature of the CO.sub.2 drops to approximately -90.degree. to -100.degree. F. Such a single orifice nozzle is known to have a low efficiency and results in a large amount of CO.sub.2 remaining in the liquid state after passing through the orifice. At the lower pressure on the downstream side of the injection nozzle, this excess liquid can result in converting some of the solid CO.sub.2 back to liquid or to gas. At the lower pressure, such CO.sub.2 liquid quickly boils off into gas. Any CO.sub.2 in the liquid or solid state after passing through the injection nozzle is wasted.
Also known in the prior art, is a nozzle referred to as the Brody Horn which is also used for converting liquid CO.sub.2. The Brody Horn has three small orifices formed in the nozzle body. These orifices are angled outward from the center of the nozzle, at an angle to the direction of the flow, which helps to disperse the CO.sub.2 solid particles downstream of the nozzle. The Brody Horn has a higher efficiency than a single large orificed nozzle. Still, the efficiency of the Brody Horn only ranges as high as 40%, which is an industry standard.
A problem with prior art nozzles is a condition known as "snowing the nozzle" which occurs when the orifice or orifices become blocked by solid CO.sub.2. This can occur in the orifices during operation or, especially, when the flow of pressurized CO.sub.2 liquid is shut off. As the pressure drops after shut off, the liquid CO.sub.2 can flash to the solid state upstream of the nozzle, thereby blocking the nozzle and preventing immediate subsequent use.
There is the need in the industry for an injection nozzle which has a higher efficiency than 40%. When used with cryogenic cleaning apparatuses, or any other similar device which requires a large flow of CO.sub.2 solid particles, the inefficiencies of the prior art injection nozzles very quickly results in high operational cost. The costs of using such nozzles over a long period of time with a high flow of CO.sub.2 can be significant. Additional orifices located in the nozzle body, or changes in the aspect ratio of the orifice length to orifice diameter do not result in any appreciable increase in the efficiency of the injection nozzle. The present invention addresses these needs by improving the efficiency of the injection nozzle.
There is also a tremendous need for injection nozzles which do not become blocked during operation or shut down. Nozzles are needed which can reliably restart every time. The present invention also addresses these needs.